Overhead projectors are used to project transparencies. One contains a glass plate with a light source from below that reflects the image in the transparency through a lens on an arm to a screen. Transparencies are polyester films of a standard A4 paper size that allows light to pass through.
Overhead projectors are simple to use and they were the first ones to be used for business and classroom presentations. This was before the arrival of data and video projectors. The projector can be set up in 10 seconds, the arm can be bent for easy portability and weighs approximately 14 lbs. The resolution can be in the range of 4000 lumens. The OHP comes with accessories such as a portable stand so that projector can be placed in the middle of the classroom, a collapsible screen, extension cords with different lengths, and sockets.
The rental rates usually vary from $ 30 to $ 50 daily, $ 100 for a week and $ 200 for a month. Most of the university classrooms have OHPs installed. The advances in data projection for CD, DVD and laptops have reduced the demand for OHP.
Use of overhead projectors has declined with the flashy presentations of PowerPoint. Still they are often used in universities, in classrooms for lectures, and job training. They are preferred over data projectors as they are simple and do not require computers for input. The transparencies can be modified during the lecture itself, which makes it versatile. The disadvantage is that OHP is fragile and bulky. Also, care should be taken to ensure that the transparencies are clear. For instance, photocopied text book pages will look bad on the screen.
Some of the important branded overhead projectors are 3M, Bell & Howell, Dukane, and High Lumens. Specific features such as built-in lamps and image magnifiers add to the higher rental rates.